Rotary steerable drilling systems have long been used in directional drilling for hydrocarbons. In general, such systems have used either “push-the-bit” or “point-the-bit” technology. The former type of system continuously decenters the bit in a given direction, while the latter changes the direction of the bit relative to the rest of the tool. Both types of existing rotary steerable systems offer significant advantages, although both also suffer from certain drawbacks, as discussed in further detail below.
One early disclosure for a rotary steerable drilling apparatus and method dates back at least as far as 1973 and is described by Bradley in U.S. Pat. No. 3,743,034 (hereinafter “Bradley”). This disclosure covers a range of topics such as using a mud driven downhole turbine or an electric motor to drive a positive displacement hydraulic pump, the use of a universal joint to connect two shafts which can be arbitrarily and continuously articulated relative to each other, and using hydraulic pistons as actuators to continuously maintain a desired direction of offset that is constant with respect to a terrestrial datum as the tool is rotating. Since Bradley precedes the commercial application of microprocessors in down hole tools, it relies on a high speed telemetry link to the surface using wired drill pipe in which segments of insulated electrical conductor are built into each joint of drill pipe (as described by Fontenot in 1970 in U.S. Pat. No. 3,518,699) to carry electrical signals through the drill pipe to the surface in order to control the steering of the tool. Bradley disclosed controlling the angle of deflection of the bias unit by regulating the length of time of opening and closing the piston control valves, the same valves that also control the direction of drilling in this configuration, to allow greater or lesser amounts of fluid to enter or leave the pistons thereby changing the amplitude of the reciprocating motion of the pistons.
Some earlier designs of rotary steerable tools use the drilling mud and the pressure drop across the bit to actuate the bias unit mechanism, regardless of whether it is using the point the bit technique, push the bit technique, or a combination of the two. Other earlier tool designs may use a mud turbine driving an electrical alternator to generate the electric power to displace the bit and maintain angular displacement.
The rotary steerable apparatus that is the subject of this disclosure solves a number of operational limitations associated with existing rotary steerable systems. Initially, it is important to note that this disclosure encompasses two distinct inventions, both of which are described in more detail below—a dynamically variable displacement axial piston pump and a hinge joint that limits the articulation of the bit to a single degree of freedom (instead of a universal joint with 2 or more degrees of freedom), providing spatially phased coherent symmetrical bidirectional deflection of the drill bit. Both inventions may be used together but either may also be used independently of the other. The term “spatial phasing” refers to the dynamic timing of an event or action related to the articulation of the bit, as the tool is rotating, with respect to a fixed terrestrial datum such as gravity or the earth's magnetic field. The spatial phase is expressed in terms of the instantaneous rotational orientation (a tool face) of a reference mark on the tool with respect to gravity (gravity tool face) or the earth's magnetic field (magnetic tool face).
Firstly, with respect to the advantages of the dynamically variable displacement axial piston pump, using a fixed positive displacement pump down hole to generate hydraulic power works only over a very narrow range of mud flow rates. If the turbine is generating enough power at the low end of the flow range, then it will be potentially generating too much power at the upper end of the flow range unless the allowable flow range is extremely narrow, thereby restricting the ability of the tool pusher to optimize the drilling parameters for efficiency and safety without damaging the tool. The novel use of a dynamically variable displacement axial piston pump disclosed herein solves this problem by dynamically reducing the displacement of the pump per revolution to maintain a constant power output as the mud flow increases, and dynamically increasing its displacement per revolution as the mud flow decreases. Secondly, the amplitude of the bit deflections, whether static or oscillatory, can be controlled by further adjusting the displacement per revolution of the dynamically variable displacement pump, allowing for control of the amplitude of the bit articulation independent from the control of the direction of drilling as the tool is rotating, whether the objective is to maintain a constant bit offset angle independent of rotation or if the bit is reciprocating at the same frequency as the rotation of the drill collar.
As used herein, the term “dynamically variable displacement axial piston pump” refers to a hydraulic pump with a rotating cylinder, driven by a drive shaft, that can be configured with two or more pistons, symmetrically arranged in the cylinder, that reciprocate in a direction that is parallel to the axis of rotation of the cylindrical piston block. The structure of this pump is described in further detail in the following sections of this disclosure. One end of each piston may end with a “slipper cup” that contacts and slides on the face of a swash plate. The swash plate is not connected to the drive shaft. Instead, the swash plate is mounted on a separate axle, the centerline of which is orthogonal to but intersects the center line of the driveshaft. When the face of the swash plate is perpendicular to the axis of the drive shaft, this is referred to as a swash plate angle of “zero degrees.” In this swash plate position, as the cylinder block rotates, the pistons do not reciprocate and the displacement of the pump is zero. As the tilt angle of the swash plate is increased to some angle θ, the pistons begin to reciprocate, increasing the displacement of the pump according to the equation Q=QO*sin(θ), where QO=[QMAX/sin(θMAX)], where QMAX is the maximum practical displacement of the pump per revolution of the drive shaft at the maximum practical swash plate angle θMAX. The other end of the pistons are connected to the hydraulic fluid ports “A” and “B” of the pump. Depending on whether the swash plate angle is positive or negative, “A” will be the outlet and “B” the inlet, or “A” will be the inlet and “B” will the outlet, respectively. The swash plate angle can be controlled by an electrical actuator or a hydraulic actuator through a linkage that is connected to the swash plate. The position of the swash plate can be measured by an LVDT (“linear variable differential transformer”) or a simple potentiometer. In a preferred embodiment, the swash plate angle is dynamically controlled by a steering control module.
Thirdly, the use of a dynamically variable displacement axial piston pump allows for instantaneous and continuously variable control of the dog leg severity of the well bore in the curved sections without having to bypass excess high pressure fluid back to tank. For tools that use the drilling mud and the pressure drop across the bit to actuate the steering control surfaces, the actuation is typically all or none. In those cases, it is not possible to partially actuate the bit deflection. By allowing for the partial actuation of bit deflection, a finer granularity of steering adjustment can be achieved and maintained while drilling.
The second invention disclosed herein relates to a hinge joint that limits the articulation of the drill bit with respect to the tool to a single degree of freedom. As will be explained in the discussion that follows, limiting the articulation of the bit to a single degree of freedom relative to a fixed point on the tool and using the method of coherent symmetrical bidirectional deflections spatially phased relative to a fixed terrestrial datum, to control the direction of drilling, allows the use of a single axis hinge instead of a two-degree of freedom universal joint to attach the bit to the bottom of the rotary steerable drilling tool. The novel method that is required to steer the well and fully benefit from the simplified mechanics of the novel rotary steerable drilling tool is referred to as “spatially phased coherent symmetrical bidirectional deflection” of the bit. This will be explained in more detail later in this disclosure. The hinge limits the motion of the bit to a single degree of freedom. However, two degrees of freedom are required in order to steer a well towards an intended target. In the invention of this disclosure, the second degree of freedom is provided by the rotation of the rotary steerable drilling tool while drilling ahead.
A BHA or “bottom hole assembly” describes the lower or bottom section of the drill string that terminates with the bit and extends up-hole to the point just below the lower end of drill pipe. In addition to the bit, the BHA can be comprised of any number of drill collars for added weight or special purpose collars that may or may not be included such as, but not limited to: stabilizers, under-reamers, positive displacement mud motors, bent subs, instrumented drill collars for the measurement of various formation and environmental parameters (for the determination, versus depth and time, of the mixture and volume of fluids in the formation or formation lithology or formation and borehole mechanical properties or borehole inclination and azimuth), or rotary steerable tools, such as the subject of this disclosure. The components that are part of a given BHA are selected to optimize drilling efficiency and well bore placement and geometry.
The timing or spatial phasing of the bit deflections is controlled so that, to an observer that is stationary with respect to the earth, the bit is reciprocatingly deflected in the same direction for every 180° of BHA rotation. Conversely, to an observer that rotates with the tool, i.e., is stationary with respect to the tool, for each 360-degree rotation of the tool, they will see a positive bit deflection towards a fixed reference mark (a “scribe line”) followed by a negative bit deflection away from the scribe line reference mark, the two deflection events separated by 180° of tool rotation.
Other benefits of using a single degree of freedom of articulation relative to a fixed point on the collar will be explained further in the disclosure that follows. Although it is not a preferred embodiment of the invention, it should be understood that a hydraulic dynamically variable displacement pump could also be used to control downhole tools other than the rotary steerable tool described above, including but not limited to a more conventional system with multiple actuators and a pivot with multiple degrees of freedom of articulation to continuously maintain an angle of articulation of the bit in a particular direction that is fixed with respect to the earth or to control the counter rotation speed of a geostationary assembly to maintain a fixed orientation of the geostationary assembly with respect to the earth as the tool rotates.